There is always a tipping point. We tend to use Bootstrap a lot at work for the reasons you mention, but you can pretty quickly get to the point where your CSS

There is always a tipping point. We tend to use Bootstrap a lot at work for the reasons you mention, but you can pretty quickly get to the point where your CSS is complex enough that you would have been better off doing it from scratch. All frameworks are like that -- you trade off initial convenience for design constraints. When I'm doing my own projects I always write my CSS by hand because it ends up less complex in the end. I don't need to see pretty things up front like my corporate customers do.